I did not use the food colouring as it is just one more product for me to scrutinize the label of, and I am quite happy with good old chocolate!

Instructions
Preheat oven to 350f/175c.
Line two baking sheets with parchment paper. Spray the parchment paper lightly with cooking spray.
In a large bowl, whisk together the powdered sugar, cocoa powder, salt, cornstarch & baking soda
Beat in two egg whites and the vanilla extract and vinegar (and food coloring if using) until the batter is moistened.
The consistency should be a thick and fudgy batter. As all eggs are not created equal, if the batter seems too thick, add another egg white or two.
Spoon batter onto the prepared baking sheets in 2 inches apart.
Bake about 12 – 14 minutes, until the tops are shiny and lightly cracked.
Slide cookies on the parchment paper onto wire racks and let cool completely.

I must confess that on my first attempt I thought the batter was too thick and all the ingredients were not being incorporated, so I added another egg white. I was slightly horrified then when I ended up with one giant cooked cookie as the batter had expanded as it cooked rather more than anticipated! So if you think it is too thick wait until you have given it a good chance to mix… not that one giant cookie is a bad thing of course!